This application claims the priority benefit of Taiwan application serial no. 88106215, filed Apr. 19, 1999, the full disclosure of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to a semiconductor process. More particularly, the invention relates to a method for fabricating a passivation layer.
2. Description of the Related Art
The application of semiconductors involves many fields. For example, semiconductors are applicable in the field of liquid crystal display (LCD). A type of LCD, which is called a reflection type LCD, comprises a top metal layer of a typical semiconductor substrate as a light reflection layer. The metal layer can be functioned as a mirror to reflect the incident light thereon, so as to generate a virtual image needed for display.
However, a passivation layer, which is often formed to cover the metal layer as a protection thereof, degrades the reflection effect of the metal layer. This degradation including reduction of intensity of the reflected light, and the reflected light therefore has insufficient intensity for generating the desirable virtual image.
Two conventional methods for fabricating the passivation layer are as follows:
1. Sequentially depositing an oxide layer and a silicon nitride layer over a surface metal layer of a semiconductor substrate. The oxide layer and silicon nitride layer constitute a passivation layer to resist moisture and to prevent the metal layer from being scratched. The passivation layer and the semiconductor substrate to be protected are shown as FIG. 1. The substrate comprises an inter-metal dielectric layer (IMD) 10, a titanium nitride/titanium (TiN/Ti) layer 12, and the top metal layer 14. The substrate has an opening 16 that exposes a portion of the IMD 10. The opening 16 is filled up with the passivation layer consisting of the oxide layer 18 and silicon nitride layer 20.
As shown in FIG. 1, the passivation layer does not have a smooth surface, thereby degrading the reflectivity of the top metal layer 14 to only about 30%.
2. Using techniques of spin-on-glass (SOG) and chemical-mechanical polishing the SOG layer to achieve a planarization. As shown in FIG. 2, the passivation layer formed by the method comprises a conformal silicon-rich oxide (SRO) layer 38, a spin-on-glass (SOG) layer 42, a TEOS layer 44, and a silicon nitride layer 46. A substrate, which consists of an IMD 30, a TiN/Ti layer 32 and a top metal layer 34, is covered by the passivation layer. The substrate has an opening 36 that exposes a portion of the IMD 30 but that is filled up by the conformal SRO layer 38 and SOG layer 42. However, the polishing step performed on the SOG layer 42 and stopped on the SRO layer 38 often scratches the SRO layer 38. Therefore, the polishing step degrades the planarization of the passivation layer and reduces the reflectivity of the top metal layer 34.
Even if the polishing step is replaced with an etching back step, the thickness of the SOG layer 42 is not easily controlled. Therefore, the etching back step also fails to accomplish the planarization of the passivation layer and to ensure the reflectance of the top metal layer 34.
Accordingly, the present invention provides a method for fabricating a passivation layer. A metal layer is provided. An oxide layer is formed on the metal layer, followed by formation of a first silicon nitride layer, a spin-on-glass (SOG) layer, and a second silicon nitride layer over the SOG layer. The passivation layer is formed comprising the oxide layer, the first silicon nitride layer, the SOG layer and the second silicon nitride layer.
The oxide layer has a thickness of about 675 to about 825 angstroms, and is preferably about 750 angstroms thick. The first silicon nitride layer has a thickness of about 576 to about 704 angstroms, and is preferably about 640 angstroms thick. The SOG layer is formed by a coating method with a thickness of about 1010 to about 1310 angstroms, and preferably, about 1160 angstroms. The second silicon nitride layer has a thickness of about 1080 to about 1320 angstroms, and is preferably about 1200 angstroms thick.
Fabricating the passivation layer comprising the oxide layer, first silicon nitride layer, SOG layer, and second silicon nitride layer ensures the high reflectivity of the metal layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.